Avian reoviruses have been associated with a wide variety of pathologies in commercial poultry. The most economically important reovirus disease is the arthritis/tenosynovitis syndrome. This condition is characterized by swelling of the tendon sheath of the metatarsus tendon immediately above the hock joint with resulting lameness of varying degrees. Gross swelling can result in reluctance of the chicken to move. The affected tendons can become firm and fibrotic, and adhesions to the tendon sheath and skin can result in a partially non-functional tendon [Johnson et al., Avian Dis. 15:829-834 (1971)]. Tendon rupture may occur in older birds [Jones et al., Vet. Rec., 96:153-154 (1975)].
Reoviruses have also been associated with a syndrome called malabsorption or pale bird syndrome [Page et al., Avian Dis., 26:618-624 (1982)]. This intestinal condition has been characterized by stunted growth, poor feathering, loss of pigmentation, enlargement of the proventriculus, enteritis, and leg weakness. The disease has been thought to be due to poor absorption of feed nutrients as a result of reovirus infection [Hieronymus et al., Avian Dis., 27:246-254 (1983)].
Other pathologies thought to be caused by reoviruses are hepatitis, hydropericardium, ascites, pale kidneys, small spleens, pericarditis and myocarditis.
These conditions result in-economic losses due to downgrading of broilers and poor performance in breeders which has been conservatively estimated at 15 million dollars per year.
Live vaccines in the United States have been developed from various passage levels of one arian reovirus strain, S1133, isolated and characterized by van der Heide from a field case of tenosynovitis. The strain was grown serially 235 times in the chorioallantoic membrane (CAM) at 37.degree. C. and then 65 times in chicken embryo fibroblast (CEF) at 32.degree. C. An additional 135 passages were carried out at 37.degree. C. in CEF [van der Heide et al., Avian Dis., 27:698-706 (1983)].
Current vaccination programs in breeders recommend vaccinating chickens at young age (7-14 days) with a mild vaccine derived from a high passage S1133, a live vaccination at 6-11 weeks of age with a slightly more virulent S1133 vaccine virus derived from a lower passage, with possibly a third live vaccination with the same slightly more virulent S1133 vaccine, followed by vaccination with killed products.
In broilers, vaccination is also performed as young as possible with a mild vaccine.
Vaccination at 1-day of age is not recommended with current vaccines due to the possibility of interference of reovirus with Marek's vaccination. Interference with Marek's vaccine has prevented the administration of the known reovirus vaccines at the same time as other immunizations, and has required a second series of vaccinations. Rosenberger, J. K., Western Poultry Disease Conference Proceedings, pp. 50-51, (1983).
Moreover, current vaccines have been found to persist in the tendon of vaccinates (Montgomery, R. D. and Maslin, W. R., Avian Dis., Vol. 32, pp. 461-468, 1988) with the potential to multiply and cause arthritis or other leg problems as well as to be transmitted through the egg to the offspring.
An ideal vaccine would be one which is nonpathogenic, which would not persist in the chicken and which will not interfere with Marek's vaccines and thus could be administered at 1-day of age.